As explained in the above mentioned copending application, a time base corrector [TBC] unit is a video processing device. It is a unit which can be built into a video tape recorder or can be used separately with such recorder. When video tapes are edited, the tapes must be played. Each time a tape is played on a video tape recorder, it is necessary to adjust several parameters of the associated TBC, such as video gain, chroma gain, setup and hue, so that the values of these parameters used in playing the tape the first time will be unaltered when the tape is played again. If the values are changed, the tape cannot be properly edited.
All TBC units are nominally identical. However, because of manufacturing tolerances and the like, no two units have identical electrical characteristics so that parameter settings used for play of one tape on one unit will differ from the settings needed to play the same tape on another unit in such manner that the tape can be properly edited.
The copending application discloses both a new and improved method and a new and improved apparatus for overcoming these differences in electrical characteristics, thus enabling a tape to be played for editing on any one of a plurality of TBC units without changing parameter settings each time this tape or any other tape is to be so played. In particular, an offset memory contains a separate set of offset signal values associated with each unit and a tape memory contains a separate set of parameter values associated with each tape. Then, by using data obtained from both memories, any one of these tapes can be played in any of the units with identical results.
However, it is often necessary to play a tape having a known set of tape parameters in the tape memory on a TBC unit having unknown offset signal values. In addition, a TBC unit may also be subject to drift, that is, over an extended period of time such as several months, the electrical characteristics of the same unit may change somewhat with time. As a consequence, even when the unit previously had known offset signal values as determined in the manner taught in the copending application, the same tape played on the same unit after drift has occurred will not play properly until a new set of offset signal values are obtained for the unit in order to compensate for drift.